Free radical carbonylation of 1,4-cis-polybutadiene

Full text of DOI:10.1021/ja9543053

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J. Am. Chem. Soc. 1996, 118, 7223-7224
Free Radical Carbonylation of 1,4-cis-Polybutadiene
7223
C. Chatgilialoglu,*,1 C. Ferreri,1,2 and A. Sommazzi3
I.Co.C.E.A., Consiglio Nazionale delle Ricerche
Via P. Gobetti 101, 40129 Bologna, Italy
Istituto Guido Donegani, Via Fauser 4
28100 NoVara, Italy
ReceiVed December 28, 1995
The copolymerization of olefins with carbon monoxide has
been studied extensively over the last four decades.4 This
reaction is used, on both industrial and laboratory scales, for
the synthesis of polymeric materials containing ketonic groups.
These polymeric materials are especially important due, at least,
to the following reasons: (i) CO is particularly plentiful and
inexpensive; (ii) the carbonyl chromophore renders these
copolymers photodegradable; (iii) polyketones are useful starting
materials for other types of functionalized polymers with
specialized properties; (iv) some of them show high mechanical
strength which results from a high crystallinity. In the early
years the reactions were performed under free radical conditions
in which peroxides, hydroperoxides, and azo compounds, as
well as γ-rays, were widely used.5 With the discovery that
transition metals and their complexes catalyze the copolymer-
ization of olefins with CO, the radical-initiated reactions were
largely neglected.6 The radical methodology produces random
olefin-carbon monoxide copolymers (olefin:CO > 1),7 whereas
with the transition metal catalysts, a regular structure with
alternating olefin and carbon monoxide units (olefin:CO ) 1)8
is obtained.
In the last decade, chemically modified polymers have gained
increasing importance in the manufacture of rubbers and plastic
materials. Unsaturated polymers are particularly suitable for
such transformations. In a recent patent,9 some of us described
the free radical carbonylation of polydiene containing adjacent
structural units derived from 1,4-cis polymerization of conju-
gated dienes. Polyketonic structures are also produced by this
methodology. In fact, the new class of polymeric materials
which results from the carbonylation contains randomly dis-
tributed repeating cycloketonic units along the chain. The origin
of different ketonic moieties is the most intriguing aspect of
this procedure, the mechanism of which also will be considered
in this communication.
Figure 1. IR spectra of polymers A and B between 1500 and 2000
cm-1, obtained using 60 and 100 atm of CO, respectively.
the reaction time (ca. 6 h). Experiments were performed at
temperatures of 80 °C (initiator: AIBN), 90 °C (dibenzoyl
peroxide), and 140 °C (di-tert-butyl peroxide) and carbon
monoxide pressures of 60 and 100 atm. After normal workup,
1
the polymeric material was characterized by IR, 13C, and H
NMR spectroscopies and elemental analysis. An important
feature of this polymer modification is that the content of
carbonyl units11 and the ratio of cyclopentanone/cyclohexanone
depend strongly on the experimental conditions, i.e. CO pressure
and reaction temperature.
The IR spectra between 1500 and 2000 cm-1 of two samples
are reported in Figure 1. Polymers A and B contain 5% and
20% CO by weight, respectively.12 These products were
obtained by using CO at 60 atm and AIBN or CO at 100 atm
and dibenzoyl peroxide, respectively. The IR spectra of these
polymers show characteristic carbonyl bands which are typical
of cycloketone structures. Such bands are usually centered
around the frequency of 1730 cm-1 for cyclopentanone units
and 1700 cm-1 for cyclohexanone units. Ketonic structures,
either from larger rings or from acyclic arrangements, display
IR bands centered between 1650 and 1700 cm-1
.
In order to control both the incorporated amount of carbon
monoxide and the ratio of cyclopentanone/cyclohexanone and,
consequently, the properties of the resulting polymeric mate-
rial,13 mechanistic information regarding the elementary steps
is needed.
The elementary steps that we conceived for the modification
of 1,4-cis-polybutadiene are outlined in Scheme 1.14 Radical
1, initially generated by small amounts of the radical initiator,
adds to carbon monoxide to form the acyl radical 2 that
undergoes either a 5-exo-trig or a 6-endo-trig cyclization to form
radicals 3 and 4, respectively. Radical 4, in turn, can either
add to another CO molecule followed by a 6-exo-trig cyclization
or undergo a 5-exo-trig cyclization to give carbonyl-containing
The modified polymers (5) were prepared by radical-initiated
carbonylation of 1,4-cis-polybutadiene.10 In particular, 1.3 L
of a toluene solution containing 1-4 g of polybutadiene and
the radical initiator was charged under a nitrogen stream in a 2
L AISI steel autoclave, equipped with mechanical stirring means.
Carbon monoxide was then added at constant pressure during
(1) Consiglio Nazionale delle Ricerche.
(2) On sabbatical leave. Permanent address: Dipartimento di Chimica
Organica e Biologica, Universita` di Napoli, Via Mezzocannone 16, 80134
Napoli, Italy.
(3) Istituto Guido Donegani.
(4) For reviews, see: (a) Sen, A. AdV. Polym. Sci. 1986, 73/74, 125. (b)
(11) CO content ) [O2 content (sample)] / 0.571. The O2 content was
determined by measuring the thermal conductivity of gases deriving from
the pyrolysis of the sample. Therefore, the CO content is an indirect
measurement of the amount of structural cycloketonic units.
Sen, A. Acc. Chem. Res. 1993, 26, 303.
(5) For the initial work, see: Brubaker, M. M. Coffman, D. D.; Hoehn,
H. H. J. Am. Chem. Soc. 1952, 74, 1509.
(6) For recent work, see: (a) Sperrle, M.; Consiglio, G. J. Am. Chem.
Soc. 1995, 117, 12130. (b) Jiang, Z.; Boyer, M. T.; Sen, A. J. Am. Chem.
Soc. 1995, 117, 7037. (c) Jiang, Z.; Sen, A. J. Am. Chem. Soc. 1995, 117,
4455. (d) Forbes, M. D. E.; Ruberu, S. R.; Nachtigalliva, D.; Jordan, K.
D.; Barborak, J. C. J. Am. Chem. Soc. 1995, 117, 3946.
(7) Polyketones with a low carbon monoxide content are currently used
as photodegradable plastics. For example, see: (a) Bremer, W. P. Polym.
Plast. Technol. Eng. 1982, 18, 137. (b) Leaversuch, R. Mod. Plast. Int.
1987, 17, 64.
(12) Although the IR spectra proved to be the most diagnostic, 13C and
1H NMR spectra also showed some important structural features, in
particular at higher CO incorporation. The NMR spectra of polymer B are
available in the supporting information.
(13) DSC analyses of the starting 1,4-cis-polybutadiene and the carbo-
nylation products (polymers A and B) are the following (rate 20 °C/min):
polybutadiene Tg ) -110 °C, Tc ) -71 °C, and Tm ) -8 °C; polymer A
Tg ) -108 °C, Tc ) -60 °C, and Tm ) -16 °C; polymer B Tg ) -97 °C,
Tc ) 153 °C, and Tm = 300 °C. It is evident that for the polymer with the
higher incorporation of CO (20%), the thermal properties of the material
change substantially. The thermograms are available in the supporting
information.
(8) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663.
(9) Sommazzi, A.; Cardi, N.; Garbassi, F.; Chatgilialoglu, C. U.S. Patent
5,369,187, 1994.
(10) The polybutadiene which has been used in this work contains 1,4-
cis units of more than 98% and an average viscosimetric molecular weight
of 200 000 as measured in toluene at 30 °C.
(14) (a) Ryu, I; Sonoda, N.; Curran, D. P. Chem. ReV. 1996, 96, 177
and references cited therein. (b) Ryu, I.; Kusano, K.; Hasegawa, M.; Kambe,
N.; Sonoda, N. J. Chem. Soc., Chem Commun. 1991, 1018.
S0002-7863(95)04305-8 CCC: $12 00 © 1996 American Chemical Society

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7224 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Scheme 1
Communications to the Editor
Scheme 3
Scheme 2
12 were an intermediate in the ring expansion, the rate constant
for reaction 12 f 9 would have to be larger than 107 s-1 at
80 °C.
bicyclic structures. Many repetitions of these elementary steps
allow for the formation of polymer 5 which contains blocks of
both cycloketonic groups and 1,4-cis-polybutadiene. As can
be seen from Figure 1, an increase of the CO pressure leads to
an increase in the cyclopentanone/cyclohexanone ratio. There-
fore it is necessary to invoke a rearrangement of 3 to 4 in order
to explain the ratio recovered in the experiments. A search of
the literature15 revealed the mechanistic picture illustrated in
Scheme 2 for the cyclization of acyl radicals and the related
one-carbon ring expansion. Since this overall mechanism is
far from being understood, it brought us to design and perform
the following model experiments.
To a degassed solution of either erythro-trans or threo-trans
thionocarbonate 616 (0.288 g; 0.80 mmol) and AIBN (0.013 g;
0.080 mmol) in benzene (5.5 mL), (TMS)3SiH (2.46 mL; 8.0
mmol) was added. The mixture was then heated at 80 °C for
ca. 1 h until the starting thiocarbonate disappeared (TLC
monitoring). GC analysis, using an internal standard, of the
crude reaction mixture revealed the formation of cyclopentanone
8 (trans isomer) and cyclohexanones 1017 in a 6:1 ratio and
quantitative yield. The mechanism for this reduction is outlined
in Scheme 3. Tris(trimethylsilyl)silyl radicals remove the
thionocarbonate moiety to form the secondary alkyl radical 7.16
This intermediate either abstracts a hydrogen atom from the
silane to give 8 or rearranges to 9 followed by hydrogen transfer
to afford 10. The absence of compounds 13 and 1517 excludes
the formation of the acyl radical 12 as an intermediate for the
following reasons. The 1.0 M concentration of the silane
remains essentially constant under our experimental conditions,
and since the rate constant for the reaction of secondary alkyl
radicals with (TMS)3SiH is 4.3 × 105 M-1 s-1 at 80 °C,18 we
estimate ke (the rate constant for ring expansion) to be nearly 7
× 104 s-1 by applying free radical clock methodology.19 From
kinetic data20 the rate constants for reactions 12 f 13 and
12 f 14 were calculated to be 7.2 × 104 M-1 s-1 and 3.7 ×
105 s-1, respectively, at 80 °C. Therefore, if the acyl radical
In order to measure the rate of the reaction 12 f 9, we
performed the following experiment. To a carefully degassed
solution of phenylseleno ester 11 (0.200 g; 0.56 mmol) and
AIBN (0.009 g; 0.056 mmol) in benzene (1.05 mL), tributyltin
hydride (0.75 mL; 2.8 mmol) was added. The mixture was then
heated at 80 °C for ca. 1 h until the starting thiocarbonate
disappeared (TLC monitoring). GC analysis, using an internal
standard, of the crude reaction mixture gave cyclopentanone 8
(yield 47%) and cyclohexanone 10 (6%) together with the
aldehyde 13 (36%) and the alkene 15 (11%). The reactions
expected to take place upon treatment of phenylseleno ester 11
with Bu3SnH are also shown in Scheme 3. The acyl radical
12, generated by the reaction of stannyl radical with phenylse-
leno ester 11, disappeared following four independent paths:
(i) hydrogen abstraction from the Bu3SnH to give 13; (ii)
decarbonylation to give 14; (iii) 5-exo-trig cyclization to give
7; and (iv) 6-endo-trig cyclization to give 9. At the high
concentration of Bu3SnH used in the experiment above, the rate
constant for the path 7 f 8 is at least 50 times faster than ke.
Taking 3.7 × 105 s-1 as the rate constant for the decarbonylation
(kd) at 80 °C,20 we calculated the rate constants for the 5-exo-
and 6-endo-cyclizations (i.e. k5 and k6) to be 2 × 106 and 2 ×
105 s-1, respectively.
In conclusion, the radical-based reaction of 1,4-cis-poly-
butadiene with carbon monoxide afforded a new polymer
containing repeating cycloketonic units. The model studies we
performed prove that the formation of the six-membered ring
is due either to direct cyclization of the acyl radical intermediate
(k6 ) 2 × 105 s-1) or to the ring expansion (ke ) 7 × 104 s-1
)
of the previously formed five-membered ring (k5 ) 2 × 106
s-1). By taking into account the mechanistic information
obtained for the elementary steps of this transformation, it is
possible to perform the reaction with control of the incorporated
amount of carbon monoxide and the ratio of cyclopentanone/
cyclohexanone which, consequently, will be reflected in the
properties of the resulting polymer.
Acknowledgment. We thank Drs. N. Cardi and F. Garbassi for
helpful discussions. C.C. and C.F. thank the Progetto Strategico del
CNR “Tecnologie Chimiche Innovative” and MURST 60%, respec-
tively, for partial support of this work.
(15) (a) Dowd, P.; Zhang, W. Chem. ReV. 1993, 93, 2091. (b) Brown,
C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J. Aust. J.
Chem. 1995, 48, 363 and references cited therein.
(16) The diastereoisomerism is expected to be lost in the formation of
the corresponding carbon-centered radical.
(17) In comparison with the retention time of their authentic samples.
(18) Chatgilialoglu, C.; Dickhaut, J.; Giese, B. J. Org. Chem. 1991, 56,
6399.
Supporting Information Available: Experimental procedures and
compound characterization data (14 pages). See any current masthead
page for ordering and Internet access instructions.
(19) Newcomb, M. Tetrahedron 1993, 49, 1151.
(20) Chatgilialoglu, C.; Ferreri, C.; Lucarini, M.; Pedrielli, P.; Pedulli,
G. F. Organometallics 1995, 14, 2672.
JA9543053